This invention relates to liquid heaters in which liquid flowing therethrough is heated by hot gas also flowing therethrough. The liquid may be water, but may of course be another liquid.
Liquid heaters of the above general kind are well known. Usually, the liquid flows through at least one heat conducting pipe over which hot gas passes to heat the liquid therein. Problems which can arise with known water heaters of this kind include thermal shock imparted to the structure of the heater by temperature differences which occur within the heaters and corrosion caused by the hot gas or the liquid being heated.
It is therefore an object of the invention to provide an improved water heater of this kind in which such problems are minimized.
According to the invention, a hot gas water heater has a housing containing at least a pair of concentric spaced coils through which liquid to be heated is sequentially passed. Each coil has a helical tube with adjacent turns being connected by a web member such that the tube and web member form an imperforate coil wall. Hot gas is passed sequentially between the coils such that its flow is co-current with liquid flow in one adjacent coil and counter-current with liquid flow in another adjacent coil.
The hot gas may flow between a pair of adjacent coils in co-current flow with the liquid flow in the inner coil and in counter-current flow to the liquid flow in the outer coil.
The housing may have a central hot gas inlet through which hot gas passes to a central passage in the inner coil and a gas outlet which receives gas from a passage between an outer coil and a coil adjacent thereto.
At lease one coil may taper towards an adjacent coil in the direction of gas flow therethrough. There may be at least three or at least four concentric coils provided.